HELP! Interesting molecular transport argument...
kondo at buphy.bu.edu
Tue Oct 25 17:39:47 EST 1994
Jason Comander (jcomand at scws6.harvard.edu) wrote:
: A friend and I are having a huge disagreement over a
: fundamental concept in biochemistry and enzymology- Can a cell do
: anything to speed up an oxygen molecule's transit from the cell
: membrane to a mitochondria in the middle of the cell? I would
: appreciate any of your input to help us work this out.
I have to agree with you, Jason that a diffusible oxygen/ferry
would not speed up the transit time of the oxygen unless it was
actively transported, such as organelle transport along microtubules.
However, such active transport (depending on ATP) uses energy, so what
the advantage may be nil and perhaps harmful in energy restrictive states.
There should be an oxygen concentration gradient, which favors oxygen
transport to the mitochondria (evidence of such a gradient? don't
have it handy) One thing to note: isolated mitochondria have been
shown to happily function in the presence of only 1 torr oxygen.
: Argument #2. Getting frustrated with the above argument, we
: managed to create another one about concentration gradients. Let's
: say that many copies of our hypothetical enzyme can store up a
: large number of oxygen molecules near the mitochondria. Assuming
: our system is at equilibrium, there would be no oxygen
: concentration gradient, because entropy will equalize the
: concentration of oxygen to the same value everywhere. The question
: is: When calculating the concentration of oxygen molecules in the
: area around the mitochondria, do the bound oxygen molecules get
: counted? i.e. Can a free oxygen molecule entering the neighborhood
: of the mitochondria "see" that there are really hundreds of (bound)
: oxygen molecules all around it, and that it shouldn't be there
: because the oxygen concentration is high? even though the bound
: oxygens are hidden deep within an enzyme?
: Jason Comander
I would say that the free oxygen concentration is the important
parameter. The concentration of bound oxygen affects the [free oxygen]
through the dissociation reaction of oxygen and its buffer molecule.
Assuming a very low dissociation constant, then most of the oxygen
will remain bound and the free concentration will be low, so more
free oxygen will diffuse to the mitochondria (assuming of course that
the concentration outside the cell and in the cytoplasm is higher). Consider
calcium release from the sarcoplasmic reticulum. Far, far greater free
calcium is released by the SR than can be accounted for by the increase in
[Ca2+]. Most of it becomes bound to protein, so the gradient from the
SR to the cytoplasm is probably always downhill. I would guess that
such buffer proteins for oxygen would help, if the electron transport chain
does not 'use' oxygen quickly enough (a nearby oxygen reserve would be
kondo at buphy.bu.edu
Cardiac Muscle Research Lab
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